with requests.Session() as c:
url = 'https://firefly.clevedonschool.org.uk/login/login.aspx?prelogin=http%3a%2f%2ffirefly.clevedonschool.org.uk%2f'
USERNAME = 'usr'
PASSWORD = 'pass'
c.get(url)
login_data = dict(username=USERNAME, password=PASSWORD)
c.post(url, data=login_data, headers={"Referer": "https://firefly.clevedonschool.org.uk"})
tasks = c.get("https://firefly.clevedonschool.org.uk/tasks")

print tasks.content

soup = bs(urllib2.urlopen("https://firefly.clevedonschool.org.uk/tasks").read(), "html.parser")

tasks_list = soup.findAll('li', class_="ff-task")

print tasks_list